Only speaking for my area in the rust belt, not many. Even cars and trucks from the 1990's are getting rare to see on the road.I was wondering about how many 1980's vintage cars are still out there.
Also most IAC are controlled by a ECM. The IAC stand alone controller would also need at least a few of the signals that the ECM needs. I can't imagine an owner of an old car, that needs it for transportation, being able to understand how to rewire his car to make it work.